Myosin is responsible for coupling the free energy of ATP hydrolysis to force production in muscle contraction. Relatively little is known about the changes which occur in the myosin structure during energy transduction, primarily due to a scarcity of knowledge on the structure of the myosin globular head where the actin binding site and the ATPase active site reside. Two recent observations are of interest here: 1.) Spectroscopic and kinetic techniques demonstrate that the myosin ATPase head exists in at least two native structural states, in the presence and absence of nucleotides and actin. It is not known if differences between these states are global, if they are confined to one or more domains, or if they represent movement of domains. 2.) A number of experiments have indicated that the myosin head may be composed of three domains. This application proposes to use differential scanning calorimetry (DSC) to determine the existence of independent cooperative domains, and to more precisely define the multistate nature of skeletal myosin heads. There are three primary goals of this proposal: 1.) to determine by calorimetry if the transitions between native myosin states are two-state transitions, 2.) to determine if the myosin head is more than a single unit or domain, and 3.) to determine if myosin heads and S- 2 are linked. The first goal will provide useful information on the structure of S-1 by indicating whether or not there are two fundamental, macroscopic, native states for myosin heads. The second two goals will provide useful information on the structure of S-1 and HMM and possible communication between the heads of myosin, and between each head and S-2. In order to advance our understanding of the process of energy transduction in the myosin ATPase, it is important that we define structural transitions in myosin and the energetics of these transitions. In addition we need to ascertain the existence of cooperative domains, and the coupling of domains within myosin. This proposal addresses both of these areas.